1. Field of the Invention
The present invention relates to a material for 1.5 .mu.m wide-band optical isolators.
2. Related Art Statement
Recently, there has been a tendency to use long wavelengths in a 1.5 .mu.m band instead of conventional wavelengths in a 1.3 .mu.m band for optical transmission technology. This is because optical transmission loss of an optical fiber is remarkably reduced in the case of 1.5 .mu.m band, so that the light can be easily amplified. Further, there has been a strong demanded for increasing optical transmission capacity by using multiple wavelength optical transmission technology. Therefore, 1.5 .mu.m wide-band optical isolators have rapidly become necessary.
In the multiple wavelength optical transmission, n kinds of laser rays having different wavelengths are modulated into input signals, which are passed through an optical isolator, coupled into an optical fiber by using an optical wave synthesizer, and transmitted together. On a receiving side, the transmitted light is divided into the above laser rays, and intended signals are taken out. The optical isolator is an optical element which functions to pass optical rays in one direction but interrupt them in the reverse direction. Particularly in the case of multiple wavelength optical transmission, the optical isolator to be inserted between the optical fibers is required to function well with respect to a number of multiple optical rays. Thus, the optical isolator must be of a wide band range.
The operating principle of the above optical isolator will be explained. Main constituent elements of the optical isolator are a polarizer, an analyzer and a Faraday rotation element. If light is introduced into the optical isolator in a normal direction, only given optical ray components pass the polarizer. When the polarized optical ray components pass the Faraday rotation element, a deviation angle of these components is turned by 45.degree.. Then, the components pass the analyzer. On the other hand, if the light is introduced into the optical isolator in a reverse direction, only other given optical components pass the analyzer. When these polarized optical ray components pass the Faraday element, the polarized optical ray components are rotated by 45.degree. in the same direction as in the normal direction. Therefore, even if the polarized optical ray components are introduced into the polarizer, the optical rays are interrupted by this polarizer because the deviated direction of the optical rays introduced is orthogonal to the deviating direction in the polarizer.
However, the Faraday rotation angle generally varies depending upon the wavelength. Therefore, as the wavelength of the light source changes, the rotation angle of the optical rays given by the Faraday element deviates from 45.degree.. As a result, the degree at which the light introduced in the reverse direction is interrupted decreases, so that the performance of the optical isolator becomes worse. Therefore, it is necessary to reduce changes in the rotation angle of the Faraday element with changes depending on the wavelength. Particularly, since multiple wavelengths are used in the 1.5 .mu.m band in the case of the 1.5 .mu.m wide-band optical isolator, it is necessary to prevent substantive change in the Faraday rotation angle owing to changes in the wavelength in this wide band range.
Disclosed in an article entitled "Magneto-optical properties of (TbBi).sub.3 Fe.sub.5 O.sub.12 and its application to a 1.5 .mu.m wide-band optical isolator" in "J. Appl. Phys." Vol. 70(8), Oct. 15, 1991 is a bismuth-substituted terbium-iron garnet single crystal having a composition of Bi.sub.x Tb.sub.3-x Fe.sub.5 O.sub.12 as a material for 1.5 .mu.m wide-band optical isolators. This garnet is a garnet produced by a flux process. However, a uniform composition cannot be realized by this producing process, and the production requires a long time and mass production thereof is not possible.
The present inventors had examined the production of the garnet having the above-mentioned composition system by a liquid phase epitaxial process. However, only thin films having a maximum thickness of around 500 .mu.m can be produced by this process. However, it is necessary to form a film having a thickness of about 1.5 mm to about 2.0 mm as the material for the 1.5 .mu.m wide band optical isolator.